Identifying genome associations with unique mastitis phenotypes in response to intramammary <i>Streptococcus uberis</i> challenge

Mastitis, the inflammation of the mammary gland, dramatically decreases dairy industry revenues and milk quality, making improved control and prevention methods a goal of the industry. Prior research has observed variation in response to experimental challenge with regards to inflammation indicators, S. uberis concentrations, and the need for antibiotics. To determine possible causes of the observed variation following experimental S. uberis intramammary challenge, we performed genome association analyses (N = 34 – 36 Holstein dairy cows), a linkage analysis, and S. uberis milk inoculation (N = 21 Holstein dairy cows) trials. Association analyses were done using Illumina’s BovineSNP50 BeadChip and 10 novel phenotypes developed using S. uberis challenge data. A total of 52 SNPs were significantly associated (p < 9.34x 10^-5) with our novel phenotypes, with 17 SNPs in regions with prior evidence for mastitis or mastitis associated traits. Linkage analyses were performed using Haploview on BTA 2 to identify loci in linkage disequilibrium with the IL-8 receptor, CXCR1, which has demonstrated importance for neutrophil function critical for mastitis clearance. We determined that CXCR1 is part of two separate haplotype blocks, one that is 13 kb and the other is 440 kb. Between association and linkage analyses, 44 potential candidate genes were identified, where the majority have functions related to inflammation/ immunity or gene expression regulation. S. uberis milk inoculation trials were conducted in aseptic milk collected within 7 days post-calving from clinically healthy cows with differing CXCR1 haplotype combinations. Milk samples were inoculated with three strains of S. uberis (UT366, UT387, and UT888) in vitro and growth was monitored for 10 hours. Both S. uberis strain and CXCR1 haplotype combination significantly influenced the rate of S. uberis growth in milk (p < 0.0001) suggesting an inhibitory or permissive factor towards S. uberis growth may exist with concentration varying dependent on CXCR haplotype concentration. Further investigation of the identified candidate genes and proteomics analysis of milk factors could identify more efficient or better genetic selection methods or novel preventative/treatment compounds to combat S. uberis mastitis.